1. Field of the Invention
This invention relates to an image signal processing device arranged to process an image signal.
2. Description of the Related Art
A camera of the kind using a video floppy disc is known by the name of a still video camera (hereinafter referred to as an SV camera). The SV camera is arranged to magnetically record a still image on a video floppy disc in accordance with a current TV system such as the NTSC or the PAL system. However, a level attainable in accordance with the NTSC system hardly permits the obtaining of an image with an adequate picture quality by printing the image which is magnetically recorded on the floppy disc.
To solve this problem, an SV camera called a compatible high-definition SV (hereinafter referred to as CHSV) camera has been proposed.
The CHSV camera is arranged as follows. Two frame amounts of an image signal formed in accordance with, for example, the NTSC system (or the PAL or SECAM system or any other current TV system) are arranged in an offset state. The two frame amounts of image signal are recorded in four tracks. At the time of reproduction, the recorded image signal is first sampled. The sampled signal is temporarily stored in a memory. Then, an amount of information which is four times as much as an amount obtainable in accordance with the NTSC system can be equivalently reproduced by interpolating the recorded image signal with image information which is not recorded on the video floppy disc by means of an interpolation filter or the like.
In the CHSV camera, as is apparent from the above description, the sample value of the image signal obtained at the time of recording must be accurately restored at the time of reproduction. For this purpose, a method called a "sample-value analog transmission" method is employed. This method must be carried out to satisfy the condition of having no interference between codes according to the Nyquist theorem.
Further, the phase of resampling to be made at the time of reproduction must be accurately adjusted to the phase of the image signal. This requirement is met by means of a high-precision time base corrector (TBC) with some phase reference. As to the sample-value analog transmission, a multiple sub-Nyquist sub-sampling encoding (MUSE) method is employed. The details of the method are as described in the literature relative thereto and are therefore omitted here.
In the CHSV camera, a phase reference pulse signal called an HIT (horizontal interval test) pulse signal is employed. The HIT pulse signal, the pulse width of which corresponds to the sampling period of the image signal, is added to the image signal in a position located at the left end of the image plane. The position (phase) of the peak value of the HIT pulse signal is used as the phase reference. It has been practiced to generate the HIT pulse signal by a pulse generator and to add the HIT pulse signal in a state of an analog signal to the image signal which is outputted from an image sensor.
The HIT pulse signal is added in the state of an analog signal at an extremely high speed. Therefore, the adding process causes not only an increase in power consumption but also overshooting, undershooting or the like in the waveform of the HIT pulse signal. After the occurrence of such overshooting or undershooting, the waveform of the HIT pulse signal tends to be distorted when it passes through a low-pass filter (LPF). In such a case, the position (phase) of the HIT pulse waveform comes to deviate from the phase of the image signal. The use of the HIT pulse signal which is added in such a distorted state as a phase reference signal detracts from correct reproduction of the image signal, because accurate reproduction of the sample value is prevented by the phase deviation of resampling.